[Obertelli]

Mine is a UK model, so doesn't have a speed warning buzzer. In any case the noise was independent of speed. The car has only done 68k miles and there's no other symptoms to suggest a faulty alternator, and even if it was faulty I can't see why only one speaker would be affected.

For now, the fault has vanished as mysteriously as it appeared. But as there was a blocked drain tube causing a wet carpet under the left seat recently, and as on the single day the fault occurred that seat had a passenger, I am leaning towards the idea that the two facts may be connected. But the fault no longer appears even when that seat is occupied, so as it's impossible to diagnose and fix faults which aren't there I'm going to hope it was a one-off oddity and if it wasn't try and deal with it if it comes back.

Thanks very much for the replies and suggestions though.